1. Field of the Invention
The present invention relates generally to a light source apparatus equipped with a GaN type semiconductor light emitting element, and more particularly to a light source apparatus equipped with a GaN type semiconductor light emitting element that has a stray light eliminating function.
Further, the present invention relates to a method of eliminating the aforementioned stray light.
Still further, the present invention relates to an image forming apparatus for scanning a photosensitive material with light which has been modulated based on image data and forming an image borne by said image data.
2. Description of the Related Art
Presently, GaN type semiconductor lasers, which comprise an active layer formed of InGaN, InGaNAs, or GaNAs, that emit blue light are nearing practical application. Further, a so-called SLD (Super Luminescent Diode), which is a light emitting diode provided with an active layer of a striped structure formed of a GaN type semiconductor has also been disclosed, as described in Japanese Unexamined Patent Publication No. 11(1999)-74559, for example. Although this SLD does not generate laser oscillation, because the emission region is controlled by the striped configuration, this SLD can emit a green or blue light beam having a narrow emission angle and a microscopic emission radius.
These GaN type semiconductor lasers can be employed advantageously in apparatuses, especially color image forming apparatuses, for forming an image borne by image data by scanning a photosensitive material with a light modulated based on said image data, for example, as a light source that emits blue light.
However, according to these GaN type semiconductor light emitting elements (including both a semiconductor laser and a light emitting diode), the stray light characteristically emitted by the semiconductor material is more easily generated. Hereinafter, this phenomenon will be explained in detail.
With regard to an SLD or a semiconductor laser of a configuration comprising a layer of AlGaInP, AlGaAs, InGaAsP or the like formed on a GaAs substrate, the GaAs forming the substrate is a material which is absorptive of the wavelengths of the emitted light; further, the contact layer formed under the electrode on the side opposite from the substrate is also formed of an emission absorptive material such as InGaAs or GaAs. Therefore, even if unnecessary stray light not contained within the width of the emissions wavelength range, normally on the order of several μm, is generated, this stray light becomes absorbed by the substrate and poses no particular problems with respect to practical applications.
As opposed to this, with regard to a GaN type semiconductor light emitting element, a material transparent to light contained within the emission wavelengths, such as sapphire or SiC, is used for the substrate. As a result, a problem has been encountered wherein stray light travels to the terminal end of the substrate side or the opposing electrode side, is reflected and returned to the vicinity of the emission region and a variety of stray light patterns are formed by a plurality of reflections.
FIG. 9 shows a comparative example of the characteristics of the output of the drive current of a GaN type semiconductor laser and a semiconductor laser formed of AlGaInP. As shown in FIG. 9, the intensity of the naturally emitted light below the oscillation threshold value is markedly stronger for the GaN type semiconductor laser.
For cases in which this type of semiconductor light emitting element is driven by a current larger than the laser oscillation threshold value, because the intensity of the light emitted by the laser oscillation is of a higher magnitude in comparison to the intensity of the naturally emitted light, which becomes the origin of the stray light, this stray light normally does not cause the problem described above. However, in the case that the GaN type semiconductor light emitting element is employed as a recording light source for recording a gradation image, and driven in a low current range with direct modulation in order to make it capable of recording a high gradation image, this stray light comes to pose problems in practical application.
That is to say, if the aforementioned semiconductor light emitting element is driven by a low level drive current as described above, the generation of the aforementioned stray light becomes more likely, and in extreme cases, a light emission pattern occurs not only at the stripe portions but over the entirety of the element. The light generated in this way from the portions outside of the stripe portions cause deformation of the spot formed by focusing the recording light, which brings about a degradation of the coupling efficiency of the recording light and the optical system. If such a state is produced, it becomes difficult to accurately control the quantity of the recording light (the exposure light quantity) when a high gradation image is to be recorded, and the image quality of the recorded image is deteriorated.